Optical analysis systems, such as reflectometers, scatterometers, ellipsometers, and the like, are used in lithographic applications to analyze process layers or features of semiconductor wafers, masks, or other device structures. A lithographic scanner may be operated according to an established recipe with a selected focus value for each process layer to achieve a desired pattern performance in a resulting device structure. A focus/exposure matrix (FEM) test wafer may be used to establish focus values for the scanner recipe.
In an existing method the FEM test wafer is exposed using a production reticle and one or more wafers including significant pattern overlap across layers (i.e. short loop or full stack). A collection of critical dimension (CD) structure features in the device pattern are selected for measurement via a critical dimension scanning electron microscope (CDSEM). Patterns are measured for combinations of focus and exposure in the FEM. Data is plotted as CD versus focus, with data series by exposure. A focus value is then selected according to dimensional requirements of the set of features.
There are some problems, however, in relying on a CDSEM-based method of determining focus. For example, CDSEM throughput is generally low. CDSEM signal-to-noise ratio (SNR) degrades rapidly as CD structure size decreases. CDSEM equipment is complex, prone to recipe setup errors, and has many possible sources of uncertainty. CDSEM beams cause resist structure size to change with repeated bombardment, thereby adding uncertainty to measurements and requiring rigorous control over measurement sequence and repetition.
According to another existing method, a test target design is selected that is expected to have sensitivity of one or more CD structure parameters (e.g. line width, sidewall angle) to scanner focus change. A computing system generates a library of simulated signals, using material properties (e.g. n and k of resist, antireflective coatings, planarization films), nominal test target structure (e.g. line width, sidewall angle, line height), reasonable expected range of target structure variation, and optical analysis parameters (e.g. wavelength range, azimuth angle range, angle of incidence range, polarizations). An FEM test wafer is exposed and signals are collected from test targets for combinations of focus and exposure in the FEM. The closest match for each signal is found in the library. Corresponding structure parameters (e.g. line width, sidewall angle, line height) for the library matches are matched up with the programmed focus and exposure combinations from the FEM. Structure parameter(s) versus focus and exposure are then analyzed to determine the extents of a “combined process window”. A focus value for the scanner recipe may then be determined as the focus corresponding to the center of the process window.
The foregoing method is also burdened by various limitations. For example, library generation requires target design to be a simple repeating structure (line/space grating over unpatterned film stacks). Accordingly, the useable targets are generally limited to proxy targets in a wafer scribe area. Device structure cannot be used unless it can be modeled as a simple repeating structure on unpatterned film stacks, which is typically only possible on the very first pattern layer on memory devices. Modeling for library generation assumes structure can be represented reliably by simple trapezoidal cross-section models. Real structure profiles, however, often take on more complex shapes at focus and exposure combinations near the extents of the process window.